Marine scientists and photographers often need to know the transmissivity of the surrounding water, since knowing exactly what the underwater water visibility is is a parameter to be considered during many experiments. That which affects underwater visibility includes, among several things, the volume absorption coefficient a, and the small angle forward scattering coefficient s. These generally are identified as the transmissivity factor m, where: EQU m = e.sup.-.sup.(a.sup.+s) R ( 1)
where
m = transmissivity PA1 R = path length.
Alpha is defined as EQU .alpha. = a + s (2) EQU = (-1/R)ln m (3)
and has the standard unit of meter .sup.- .sup.1. Alphameters measure this parameter, and are used for such tasks as comparing the performance of optical systems under different water conditions, video processing to compensate for range variations, taking data for visibility prediction studies, and analysis of liquid solutions other than water.
Current methods of constructing alphameters generally use a stable light source and a photodetector whose output voltage or current is a linear function of light intensity, and, therefore, transmissivity. A logarithmic amplifier, or digital processing is used to determine .alpha..
A null balance transmissometer has been described that develops an error voltage which drives a circular linear gradient neutral density filter in an optical path to null the error voltage. The angular position of the filter is proportional to the transmissivity. Disadvantages of this device are the difficulty of manufacturing an accurate linear gradient neutral density filter, and the processing required to obtain alpha.